An X-ray computed tomography apparatus (hereinafter described as an X-ray CT apparatus) processes by a computer data acquired by an X-ray scan of a subject and creates an image of the inside of the subject. In other words, the X-ray CT apparatus exposes the subject from various directions over a number of times, detects the X-rays transmitted through the subject by a detector, and acquires a plurality of detected data. The acquired detected data is subject to A/D (analog/digital) conversion by a data acquisition system, and is transmitted to a data processing system.
The data processing system forms projection data by applying preprocessing etc. to the detected data. The data processing system executes a reconstruction processing based on the projection data and forms tomographic image data. As a further reconstruction processing, the data processing system forms volume data based on a plurality of tomographic image data. The volume data is a data set expressing a three-dimensional distribution of a CT value corresponding to a three-dimensional region of the subject.
Several scanning schemes using an X-ray CT apparatus are known. For example, in imaging using a radiocontrast agent, a pre-scan, a main scan, and a monitoring scan are known. The pre-scan is executed before the radiocontrast agent is injected. The main scan is executed after the radiocontrast agent is injected, and when the radiocontrast agent is sufficiently permeated. Usually, an image obtained by the main scan is used for diagnosis.
The monitoring scan is a method for monitoring an image created on a real-time basis while scan imaging the subject so as to acquire a correct injection timing of the radiocontrast agent.
The monitoring scan is executed to monitor the density of the injected radiocontrast agent. The monitoring scan is executed after the radiocontrast agent is injected into the subject, and over a number of times before the radiocontrast agent is sufficiently permeated. The monitoring scan is executed between the pre-scan and the main scan.
When executing a CT fluoroscopy or a monitoring scan, a diagnostic reading doctor monitors the reconstructed CT image over a comparatively long time on a real-time basis. However, since the CT image used for monitoring is almost never used for diagnosis, the CT fluoroscopy and the monitoring scan are executed with the lowest X-ray radiation dose. In this manner, the radiation dose to which the subject is exposed can be minimized. Nevertheless, it is undeniable that various negative effects are caused by the low-dose irradiation.
For example, the CT image reconstructed from the scan data acquired under a low radiation dose includes significant image noise, which makes diagnostic reading difficult. Sometimes, by applying various image corrections, the CT value of a part of the image may drop significantly, causing the image quality to increasingly deteriorate. If an image including a deteriorated portion is used, the certainty and safety of the CT fluoroscopy and the monitoring scan may be hindered.
Furthermore, the lower the radiation dose when executing the CT fluoroscopy or the monitoring scan gets, the greater dependency would be placed on the skill of an individual operator or a diagnostic reading doctor. In the above manner, since low radiation dose imaging may deteriorate the visibility of an image or hinder reproducibility of a study, technical measures of some kind are demanded.